Age-related macular degeneration (AMD), a disease causing vision loss that affects the central area of the macula, is the leading cause of blindness in people over 50 years of age (Bressler, 2004). Exudative AMD is the most severe form of AMD (Ferris et al., 1984) primarily arising from the choroidal circulation beneath the macula and characterized by choroidal neovascularization (CNV). CNV, the abnormal growth of new vessels from the choroid into the retinal pigmented epithelium (RPE) (Patz et al., 1977), is thought to lead to visual loss due to the leakage of blood and serous fluid beneath the RPE that eventually leads to loss of photoreceptors, retinal detachment and dense macular scarring (Fine et al., 2000; Campochiaro et al., 2006). Vascular endothelial growth factor (VEGF), a key factor in angiogenesis and vascular leakage (Dvorak et al., 1995) is up-regulated during the progression of CNV (D'Amore, 1994; Spilsbury et al., 2000; Anderson et al., 2002; Das et al., 2003) and has become the lead therapeutic target for the treatment of exudative-AMD.
VEGF is a complex gene that is alternatively spliced to form a family of multiple isoforms (Leung et al., 1989; Jingiing et al., 1999), each isoform differing in biological property, activity and function (Houck et al., 1991). Most cells commonly express isoforms VEGF121, VEGF165, and VEGF189, whereas VEGF145 and VEGF206 are comparatively rare. The majority of VEGF isoforms contain exons 1-5 (the exception being VEGF111 (Mineur et al., 2007)) but differing portions of exons 6 and 7 that encode heparin sulphate (HS) binding domains. Alterations in the usage of these exons changes the biological properties of alternatively spliced isoforms such as their ability to bind to cell-surface heparan-sulfate proteoglycans and release angiogenic factors (Tischer et al., 1991; Neufeld et al., 1999).
In 2002 differential splicing of the eighth exon was demonstrated from a proximal splice site (PSS) to a distal splice site (DSS) 66 bases downstream (Bates et al., 2002; Woolard et al., 2004). Alternative splicing in this region generated a second family of isofolins (VEGFxxxb), noted for their anti-angiogenic properties (Perrin et al., 2005). WO 03/02105, the contents of which are incorporated herein by reference in its entirety describes the alternatively spliced isoforms, and their therapeutic significance.
During pathological angiogenesis pro-angiogenic isoforms are selectively upregulated (Bates et al., 2002; Varey et al., 2008; Pritchard-Jones et al., 2007), suggesting VEGFxxx and VEGFxxxb may have separate regulatory pathways. These anti-angiogenic isoforms, such as VEGF165b and VEGF121b have been shown to be potently anti-angiogenic in animal models of retinal and choroidal neovascularisation, following intra-ocular injection (Hua et al 2008), and result in both endothelial and retinal epithelial cell cytoprotection (Magnussen et al 2010).
The first therapy to be FDA approved for the treatment of neovascular AMD in December 2004 was a VEGF165, VEGF189 and VEGF206 specific aptamer, Pegaptanib Sodium (Macugen). During clinical trials pegaptinib dose-dependently reduced the risk of severe visual acuity loss and slowed the progression of neovascular AMD (Gragoudas et al., 2004), but did not result in significant improvement in vision. In 2006 Ranibizumab (Lucentis), a novel humanized anti-VEGF antibody fragment, was FDA approved for the treatment of neovascular AMD. Its approval was based on the results of three clinical trials where, approximately 95% of patients treated monthly with Lucentis 0.5 mg maintained visual acuity (defined as the loss of <15 letters) and ≤40% improved vision (defined as the gain of ≥15 letters) at one year compared with 11% in the sham control treated group (Rosenfeld et al., 2006; Brown et al., 2006; Brown et al., 2009). Current treatment regimes require Lucentis administration by intra-ocular injection as often as monthly (Brown et al., 2009; Schmidt-Erfuth et al., 2011). Such intraocular injections result in increased intraocular pressure (Good et al., 2010) and a risk, albeit minor, of endopthalmitis and other severe adverse effects (Jager et al., 2004). Furthermore, bevicizumab (Avastin), an anti-VEGF antibody from which Lucentis was derived, was shown to bind VEGF165b with equal potency to VEGF165, thus targeting both pro and anti-angiogenic VEGF isoforms (Varey et al 2008).
As both the anti-angiogenic and angiogenic isoforms of VEGF are derived from the same gene, the control of isoform family is a result of the control of alternative splicing. We have recently identified some of the pathways that control the splicing of VEGF at the proximal splice site, implicating the RNA binding protein SRSF1 (Nowak et al., 2008; Amin et al., 2011) and its kinase SRPK1 (Sanford et al., 2005) as key requirements for the decision by cells to use the proximal splice site, and hence generate pro-angiogenic isoforms of VEGF (Nowak et al., 2008; Nowak et al., 2010). Knockdown of SRPK1 potently reduced VEGF mediated angiogenesis in vivo in tumours and inhibition of SRPK1 and 2 reduced angiogenesis in vivo (Amin et al., 2011).
WO 2008/11077, WO 2009/106855, WO 2010/058227, WO 2011/036429 and WO 2011/148200, the disclosures of which are incorporated herein by reference, describe therapeutic and other physiological uses of agents which direct expression in favour of the VEGFxxxb isoforms. SRPK inhibitors can in principle constitute such agents.
WO 2005063293 describes a class of SRPK inhibitors including SRPIN340 and derivatives and analogues thereof.
The development of new agents for directing expression of VEGFxxxb isoforms represents a new era not only in the treatment of, for example, neovascular AMD, but all other diseases in which VEGFxxxb is implicated.
The present invention is based in part on new small molecule inhibitors targeting SRPK1 specifically for use as anti-angiogenic agents, neuroprotective agents, agents for use in treating or preventing hyperpermeability disorders, as agents for treating pain, and as agents for reducing the risk of, or treatment of, pre-eclampsia.
The present invention is also based at least in part on the surprising finding that low molecular weight compounds known to inhibit SRPK1 (e.g. SRPIN340 and derivatives and analogues thereof) could be used topically or in dose-dependent manner to inhibit CNV progression.